Ball-grid-array (BGA) devices use a grid array of solder balls to attach contacts of the BGA device to contacts of a printed-circuit (PC) board via reflow soldering. Bad connections are a common cause of failure, and therefore it is necessary to inspect and test the connections to ensure their good quality. But as the scale of device integration advances, the number of connections to a BGA device may number in the hundreds, resulting in their being separated from each other by mere microns of distance. This close spacing makes it difficult if not impossible to test the quality of these connections by using standard x-ray techniques. Furthermore, due to their placement between (under) the device and the PC board, most of the connections cannot be reached by probes. This makes it difficult if not impossible to test the connections by measuring their electrical resistance to signals. Also, many signals are buried within multi-layer PCB boards and hence are not available externally for testing purposes.
As a last resort, testers may use the “dye and pry” technique, which relies on a liquid dye to penetrate into micro-cracks or under open solder ball connections. After the dye dries, they pry the BGA off of the PCB, inspect the solder balls for the presence of the dye and investigate problems that the dye reveals. However, this method makes it difficult or impossible to identify the true root cause of contact failures due to damage to the boards during prying. It also destroys the circuit, and hence cannot be used to ensure good quality of circuits that are intended for use after testing.
Improved thermographic testing is a non-invasive and non-destructive test method that uses thermography cameras to detect thermal anomalies, called “hot spots,” which often precede, and hence portend, equipment failure. Pulsed heat technology applies and concentrates a high-energy pulse of heat at an application point. These techniques have been used together to evaluate spot welds between joined materials such as pieces of metal (U.S. Pat. No. 6,585,146), and to inspect exposed (not covered) solder joints (U.S. Pat. Nos. 4,481,418 and 4,792,683). But, to the inventors' knowledge, it has heretofore not been known how to adapt these techniques for testing of solder joints between electronic devices, such as BGA devices, and PWBs where the solder joints are hidden from view and access (covered).